A good 500 watt PSU will do nicely. A pair of 5750's will pull about 150 watts (12 - 13 amps) at load.
One of my systems has an OC'd Q9550, 4 GB RAM, a GTX260 - a card with similar power requirements to two 5750's, a Gigabyte EP45-UD3P motherboard, 3 hard drives (2.6 TB total)and an optical, and a Soundblaster card all powered by a Corsair 750TX.
Running 3 instances of Prime95 to load the CPU and 3DMark06 to load the GPU, it pulls 375 watts from the wall as measured by my Kill-a-Watt meter. Figuring 80% PSU efficiency, the system pulls 300 watts from the PSU.